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Figure 34 B2 surface from SCF CO2 Figure 35 B2 surface from CO2 + H2O treatment at 6000 psi, 250 C for 3 hrs treatment at 6000 psi, 250 C for 3 hrs Figure 34 presents a foaming effect of rapid depressurizations on the surface with large number of pop-outs widely spread on the surface, while no real cracking happens. With 7 mole % of water added as co-solvent, we can see that in figure 35 large cracking is induced on the surface of B2 board when experiment conditions are set at 6000 psi, 250 C with 3 hours. The cracking is much larger when compared with those in figure 45 which is formed at 4000 psi, 200 C with 3 hours for PCB treatment in SCF CO2 plus 7 mole % of water. d). Summary By the comparisons at the three levels for surface cracking on B2 board between the pure SCF CO2 treatment and SCF CO2 plus water treatment, we can conclude that addition of 7 mole % of water as co-solvent has significant effect on forming of the PCB surface cracking when pressure and temperature are set above their middle levels, namely 4000 psi and 200 C. Surface cracking is presented on PCB substrates due to the fact that addition of water largely enhances the local density of SCF CO2, which enables the CO2 and H2O molecules to present in the bubble nucleation and foaming process. Accordingly, the balanced pressure inside and outside of the bubbles in CO2 plus H2O treatment is higher than that in pure CO2 condition. In the first rapid depressurization, large amount of CO2 and H2O molecules outside the bubbles are quickly released which lowers the local density of SCF CO2 outside the bubbles in the following treatment process. So the bubble surfaces are broken by the higher than bulk pressure from higher density of CO2 and H2O molecules inside the bubbles. The surface cracking are very significant for potential application of removing and recycling of PCB surface metals in which all precious metals in PCBs are included. The metals bonded in PCB substrates are only copper foils which could be easily separated and recycled after a fully delaminating of the boards. Recycling Result The supercritical fluid process is proposed as a PCB recycling technology to delaminate the PCB substrates through solubility of PCB resins in SCF CO2 and foaming effects induced by three rapid depressurizations in the treatment process. Then metals and polymer layers in the PCB substrates could be easily separated from each other and be recovered to a high purity. This section gives the PCB recycling results from delaminated PCB substrate to illustrate the effectiveness of supercritical fluid technology in PCB recycling process. 28PDF Image | Recycling Printed Circuit Board Wastes Through Supercritical Fluid Delaminating
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